To provide mobile subscribers having a subscription supported by a Public Land Mobile Network (PLMN), such as operating under the Global System for Mobile communications (GSM) standard, with multimedia telecommunications services, such as Internet, Voice-over-IP (VoIP), and many other existing and future services, separate multimedia subsystems have been developed.
The IP Multimedia Subsystem (IMS) is a standardised Next Generation Networking (NGN) architecture for telecommunication operators wishing to provide mobile and fixed multimedia services to their subscribers. IMS may operate as overlay network for existing mobile and fixed telecommunications networks and is a computer network which is built on top of another network. The term “overlay” entails that calls established in a first telecommunications network are “overlayed” to a second telecommunications network. Nodes in the overlay network can be thought of as being connected by data transfer links, each of which corresponds to a path, through many physical links, in the underlying network. IMS merges the internet with the cellular world; it uses cellular technologies to provide ubiquitous access and internet technologies to provide appealing services.
IMS provides network operators and service providers the ability to control and charge for individual services. In addition, users will be able to execute all their services when roaming as well as from their home networks.
To achieve these goals, IMS uses open standard IP protocols, defined by the Internet Engineering Task Force (IETF). A multimedia session between two IMS users, between an IMS user and a user on the internet, and between two users on the internet is established using exactly the same protocol. Moreover, the interfaces for service developers are also based on IP protocols.
When a mobile subscriber is subjected to service control in IMS, supplementary subscriber services such as, for example, Call Forwarding (CF) and Call Barring (CB), may be executed by IMS. That means that these supplementary subscriber services are no longer provisioned in the mobile telecommunications network of the subscriber, i.e. in the Home Location Register (HLR) in the case of an GSM network, for example, but only in IMS.
A mobile subscriber, for example, whose call forwarding and call barring services (and call control services) are moved to IMS, is accustomed to set, change and query his/her call forwarding and call barring services as specified by the mobile telecommunications system supporting the mobile subscriber. However, although the execution of these supplementary services is now provided by IMS, the control commands are still exchanged with the mobile telecommunications system, such as the HLR in the case of a GSM system, for example. However, the HLR does no longer hold the subscription data for the call forwarding and call barring services, because these subscription data have moved to IMS. This may result in annoying service unavailability messages or the like, providing confusion with the subscriber and may lead to unsatisfactory subscriber experience, which may give rise to a negative attitude towards a telecommunication operator and/or the services provided.
Hence, the operator is impaired in his capability to offers seamless user experience when migrating GSM call control and GSM supplementary service control to IMS, for example. Seamless user experience is considered vital for the success of GSM-IMS overlay.